The invention relates to a static memory cell, particularly to be integrated in a static memory in large numbers, comprising a semiconductor body having two transistors with cross-coupled base and collector regions, the collector regions each being connected to a load element comprising a diode. The invention further relates to a static memory having such a memory cell.
The memory cells may be formed, for example, by generally known flip-flop circuits in which the collectors can be connected, via the load elements, to a common line (for example the supply line) and in which first emitter regions are connected in common to, for example, a current source and second emitter regions are connected to read/write lines.
For a stable flip-flop it is necessary that the loop gain in the meta stable point be larger than 1. Starting from a current-voltage characteristic of the emitter-base junction i.sub.c =Io exp (qV.sub.be /kT) it can be found out that it follows from this condition that the impedance R of the load elements must be larger than (kT/qi), where k is Boltzmann's constant, T is the absolute temperature, q is the elementary quantity of charge and i is the current.
During operation, for reading into the cell a comparatively large read current, for example, 1 mA is used in connection with the access time. With these large read currents, a load element having a comparatively small resistance could be used. In the stand-by condition in which the cell is not read but in which information should remain stored, a current which is as small as possible is conveyed through the cell in order to minimize dissipation. Two contradictory requirements are imposed upon the load element, namely a low impedance in connection with the comparatively large read currents and a high impedance in connection with the comparatively small stand-by currents. In practice, the impedance of the load element can be chosen to be such that a read current ratio/stand-by current of approximately 5 can be obtained when linear resistors are used.
Larger values for this ratio are desired for obvious reasons but usually they cannot be realized when a linear resistor is used as a load element, in particular because technologically very large resistors are difficult to make accurately and inter alia because with large resistances the read currents are restricted by the available supply voltages.
A memory cell having a non-linear load element, namely a resistor with a parallel arranged diode, is described in the article "A 1024-Bit ECL RAM with 15-ns Access Time" by Ronald Rathbone et al., published in IEEE International Solid State Circuits Conference 1976, pp. 188/189. The stand-by current may have a comparatively low value (15 .mu.A). When reading the cell, the greater part of the current can be passed through the diode so that a higher value can be chosen for the resistor and hence a lower value for the stand-by current than in the absence of the diode. In the article by A. Hotter et al "A high-speed low-power 4096.times.1-Bit bipolar RAM" published on the IEEE International Solid State Circuits Conference 1978, Digest of Technical Papers, pp. 98/99, it is stated that a read current ratio/stand-by current of approximately 10 can be obtained by using such a load element. It is moreover stated in this article that, by connecting a pnp transistor across the load element, a further reduction of the stand-by current (4 .mu.A) and hence of the dissipation can be obtained while maintaining the short access time, as a result of which it is possible inter alia to considerably increase the number of memory cells in a semiconductor body without the detrimental result of a corresponding increase in dissipation.
Due to the above-mentioned stability condition, the read current ratio/stand-by current in these cell also cannot be chosen arbitrarily large.
A further disadvantage of resistance elements is that they occupy a comparatively large space in the semiconductor body, and the space requirement increases as the resistance value is made higher. Reduction of the size of the resistance elements is often difficult in connection with the electrical properties of other circuit elements, and it makes the process of manufacturing the memory device more complicated. U.S. Pat. No. 3,585,412 describes a flip-flop circuit using as load elements Schottky diodes connected in the reverse direction which behave as resistors but occupy less space than the usual resistance elements. However, these diodes show the above-described disadvantages of linear resistance elements. Moreover, as a result of the provision of a Schottky diode with the desired reverse voltage characteristic, the manufacturing process of the device generally becomes considerably more complicated.